Abuse Marth's range. When you're going in to approach with fair or nair(auto-cancelled) make sure you space yourself so that you'd be hitting Peach with the tip of your sword. With this, if she's in her shield, she takes enough stun that you can continue the pressure without being punished for it. If she gets hit by it then it's an easy combo because it's more hit stun to work with. If she crouch cancels it...well, try to dash away asap or reverse Dolphin slash or something =P
On the defense, when she's approaching behind a turnip(turnip headed for you, Peach following up with an aerial), one of your options would be to counter the turnip and she'll most likely get hit by the attack if they didn't know you were going to do that. If they start getting wise to that, you can easily wavedash backwards and swipe(jab, w/e) the turnip and just go from there.
Also, I don't know about %s specifically, but you can crouch cancel her dash attack. That can lead into a grab, f-smash, or whatever really. All that matters is that it potentially sets up for a tech chase, and tech chasing hurts Peach badly. On that note, here's some tech-chase guidelines that I follow myself.
(these would mostly apply to f-throwing Peach at low percent, but high enough to fall over into teching position. also would apply more to a Peach who's tech patterns you don't know.)
So, first you f-throw her. Then you do a short little dash forward asap after the throw(work on reacting asap out of throws!). From here, she has 7 options: no tech then get up, no tech then roll away, no tech then roll inwards, no tech then getup attack, tech in place, tech roll away, or tech roll inwards. You want to cover these options as well as you can. What I do is, I space a tipper fair/nair into the spot of her in-place actions(no tech then get up, no tech then getup attack, tech in place), and depending on your timing, it should cover all the in-place options. If you mess up the timing then you can hold your shield up to cover the get up attack option, or wavedash back to space yourself far enough not to get hit by the get up attack, but close enough to punish with f-smash. So, now, you've finished your fair/nair, and you've realized they tech-rolled away. What do you do? React asap after your aerial and follow her accordingly. Alot of Peaches like to either hold up their shield, spot-dodge, or jab after they've tech'd. So either grab to cover the shield option, space yourself within f-smash tipper distance to cover the spot-dodge option, or CC the jab into more tech chasing, lol =P. Also, dash attack can be good for chasing tech rolls and d-air tech chase for additional tech chasing, combos and sex(make sure she's not at like 0 - 10% though=P)
Other random advice:
Peach likes to 'lunge' inside Marth's range with dash attack and crap. Don't let her do that. If for some reason you messed up spacing Marth's incredible range and you see her coming in with a dash attack, do NOT jump. She will most likely catch you with the dash attack and since you're in the air, you can't CC and punish her for it and that's some free % for her. Instead, either CC and punish, or if you're too high to CC, then just activate super dash away timing and **** her with your sword. Or you can shield grab her if she didn't overshoot the dash attack to finish behind you.
More advice:
Cactuar says Marth can f-throw ->perfect wavedash tipper f-smash Peach at around 100%. I personally fail at this cuz I guess my wavedash length/timing is inferior to Cactus. ****ing Cactus.
I like to mess around with Peach sometimes and u-throw her instead of f-throw and catch her with random u-tilt -> aerial combos. I mean really, Peach is so vulnerable in the air, why the hell not?
All in all though, forcing Peach into tech chase hurts her extremely bad. Spacing and timing is key against Peach.
